1. Field of the Invention
This invention relates to an arrangement for entraining particulate materials in a carrier gas and to the application of the arrangement to a process for the production of titanium dioxide.
2. Brief Description of the Prior Art
The production of titanium dioxide may be carried out by a process in which titanium tetrachloride is oxidised in the vapour phase. By way of example, such a process may be carried out by introducing preheated titanium tetrachloride and preheated oxygen into a reactor the preheating being conducted to an extent such that the temperature which the mixture of titanium tetrachloride and oxygen would reach on mixing, if no reaction were to take place between them, would be at least 800.degree. C., and allowing reaction between the titanium tetrachloride and oxygen to take place to form titanium dioxide particles and chlorine. The mixed chlorine-containing gases resulting from the reaction may be passed from the reactor at a velocity such that the particles of titanium dioxide are entrained in the gases, the titanium dioxide separated from the entraining gases and the gases treated to enable the chlorine content thereof to be recycled to the manufacture of titanium tetrachloride. The chlorine-containing gases and entrained titanium dioxide particles may be passed through an elongated section of pipework downstream of the reactor to allow some initial cooling to take place, and the titanium dioxide may then be disentrained in primary separation means and the chlorine-containing gases may be passed through a further elongated section of pipework, which allow further cooling to take place, to further separation means comprising filter means which remove residual solids and to purification and compression means which render the gases suitable for direct use for the manufacture of titanium tetrachloride by the chlorination of titaniferous ores.
The above described process is subject to problems arising from the deposition of titanium dioxide; deposits of which are hereafter referred to as "scale" without any limitation to a particular physical form of deposit; on the exposed interior surfaces of the reaction chamber and of associated downstream pipework through which the gases and entrained titanium dioxide particles; which may have a temperature of or in excess of 1000.degree. C. at the point where the reaction between the titanium tetrachloride and the oxygen has finished; is passed while the gases are cooling to a temperature at which the titanium dioxide may be recovered. Scale formation of this nature reduces the rate at which the titanium dioxide particles and entraining gases may be cooled and may have a deleterious effect on pigment quality. Additionally, the accompanying reduction in the free cross section of the reactor and/or pipework may cause an increase in the pressure in the system. Scale formation may also occur on the interior of the pipework in which the separated chlorine containing gases are cooled and transported to the filter means with similar disadvantages.
The problem of scale deposition has been substantially solved by scouring the relevant interior surfaces with an inert particulate material. Such a material may be introduced, preferably substantially axially, into the upstream end of the reactor and or into pipework downstream thereof entrained in a carrier gas at a velocity such that, on impingement on the interior samples of the reaction chamber and/or of the pipework the interior surfaces of such reaction chamber and pipework may be kept free of scale. Alternatively or additionally, such a material may be introduced, preferably substantially axially, entrained in a carrier gas into the pipework through which the separated chlorine containing gases are further cooled and are transported to the said further separation means. Processes for the production of titanium dioxide by the vapour phase oxidation of titanium tetrachloride and the use of an inert particulate material to prevent or reduce scale deposition on interior equipment surfaces are described in, inter alia, British Pat. Nos. 1049282 and 1173592.
It is desirable to ensure that the input of inert particulate material into the system is a continuous input at or near the optimum rate to achieve the desired scouring effect. The use of a too high rate of input causes undue erosion of the interior surfaces which it is desired to protect from scale deposition resulting in a reduction in pigment quality due to contamination and involves extra costs in the provision of the extra quantity of inert particulate material and in the provision of larger handling facilities for it. Since the scale of normal commercial usage is in the range of about 40 to 1000 lbs/hr (18 to 453 kg/hr) of inert material it will be appreciated that the disadvantages mentioned above can be of considerable practical import. The use of a rate of input of inert particulate material which even temporarily falls below a critical level can allow the initiation of scale deposition. Once such deposition has started it is difficult to stop since it provides a substrate for further deposition and it is then necessary to use a disadvantageously large input of high velocity inert particulate material to ensure that, that substrate is completely removed.
The rate of flow of a particulate material in a conduit and hence the mass flow rate, may be determined by the method disclosed in British Pat. No. 1479487 according to which the arrival of a pulse of heat, injected a known distance upstream, at a thermistor is detected and the input and arrival data eletronically processed to give a suitable direct read-out.
Control of the rate of input of inert particulate material presents considerable practical problems in a commercial scale plant for the production of titanium dioxide. The gas pressure prevailing in the system is usually positive due to the generation of a back pressure by the passage of high velocity gases through the various process stages above described in a system which, to prevent venting of chlorine and other gases to the atmosphere or loss of reusable chlorine, is substantially a closed one.
In practice it is found that there is a slight pressure difference, relative to the downstream gas pressure, at filter means comprising the further separation means, of the order of 1 or 2 psi (0.07 to 0.14 bars) a somewhat greater pressure difference at the primary titanium dioxide separation means of the order of up to about 5 psi (0.34 bars) a further pressure difference of about 5 psi (0.34 bars) in the pipework through which the entrained titanium dioxide and inert particulate material is passed and a further slight pressure difference in the reactor. These pressure differences vary somewhat with process variables. The conversion to metric units of pressure above and hereafter except where the content otherwise requires is to the number of such units above atmospheric pressure. The operation of the filter means necessarily involves a periodic pressure variation of about 1 psi (0.07 bars) and may involve a greater variation due to faulty operation. Such variation in pressure is transmitted through the system back to the reactor and beyond. It can be seen, therefore, that the inert particulate material may have to be fed into the carrier gas against a varying pressure which will be over 10 psi (0.69 bars) possibly over 15 psi (1.03 bars) and even, approaching 20 psi (say up to approaching 1.37 bars) upstream of the reactor and may be over 5 psi (0.34 bars) at the point where the separated chlorine containing gases are introduced into the pipework for transport to the filter means, although at this point in the process the variation in pressure may be proportionally greater and may approach 100%. Such pressure conditions lead to uneven operation of the mechanical particulate solids feeding device and, due to this, it has been found necessary to operate with a feed rate from 100% to 300% in excess of the optimum to ensure that at no time does the rate of input of inert particulate material drop below the critical level at which scale deposition may begin. Such a high feed rate leads to problems of its own since a variation in pressure may result in a further temporary increase in the feed rate to close to that at which disentrainment might occur in the course of passage of the particulate material through the process pipework. This would involve putting the pipework temporarily out of service while the disentrained material was being removed.
Inert particulate material may be fed from a hopper by gravity flow to a point at which it is metered in suitable quantity into a flow of carrier gas and from which it is fed with the carrier gas into the reaction chamber or the pipework. A mechanical feeding device such as, for example, a helical screw feeding device is commonly used to meter the particulate material into the carrier gas. Such a feeding device is particularly prone to the problems outlined above.
It has been found that the control of the rate of delivery of the inert particulate material from the hopper to a mechanical feeding device in response to pressure variations existing downstream of that device by, for example, an electronically controlled system of sensors and valves, is insufficiently responsive to provide a satisfactory solution to the problem of variation in the rate of introduction of inert particulate material.